VI 



CHROMIDIA 191 



the nuclear material. They are then known as vegetative chromidia, 

 and have been compared to the macronucleus of Infusoria, which is 

 absorbed at the time of conjugation without playing any part in that 

 process. Often, however, the extrusion of vegetative chromidia from 

 the nucleus seems to be merely a means by which a nucleus which has 

 through unfavourable conditions become hypertrophied, gets rid of its 

 excess chromatin. The classical example of this is Actinosphaerium 

 (R. Hertwig, 1904). Under certain unfavourable conditions (either 

 starvation or over-feeding) the nuclei become greatly enlarged and 

 hyperchromatic ; the excess of chromatin may then be got rid of by 

 the emission of large quantities of it into the cytoplasm, where it 

 degenerates into brown pigment. 



The formation of vegetative chromidia in the Metazoan oocjrte L 

 has been described by many writers, and some have described it in other 

 Metazoan cells also. 



In oocyte I. it is said to take place at the beginning of the growth 

 period, characteristically at the bouquet stage, when there is to be 

 observed in many animals a deeply staining mass in the cytoplasm 

 just outside the polar surface of the nucleus (Fig. 81), e.g., Proteus 

 (Jorgensen, 1910 h), Paludina (Popoff, 1907), Gryllus (Buchner, 1909). 



This mass consists of granules or filaments which stain like chromatin, 

 and are often so closely applied to the nuclear membrane that they 

 appear to be continuous through this with the intranuclear chromatin. 

 This fact has led many cytologists to conclude that the mass above 

 referred to consists of chromatin extruded through the nuclear membrane 

 into the cytoplasm, that is to say, of chromidia. 



Often, however, the extrusion of chromidia occurs after the bouquet 

 orientation has been lost ; in this case, as in the forms where there is 

 no orientated bouquet stage, the emission takes place diffusely through 

 the nuclear membrane instead of only at the polar surface, e.g., Aricia 

 (Schaxel, 1912 ; Fig. 81). 



Often when emission is diffuse, and sometimes even when it takes 

 place from the polar surface only, in the bouquet stage (various 

 Orthoptera — Wassilieff, 1907; Buchner, 1909), the nucleolus has been 

 described as acting as an intermediary in the process, the chromatin 

 which is to be extruded being first collected into it, and thence emitted 

 into the cytoplasm. 



A precisely similar process has also been described in spermato- 

 cyte I. ; e.g., Blatta (Wassiheff, 1907 ; see Fig. 81). 



While descriptions of chromidia formation in the Metazoa have 

 mainly been restricted to oocytes and spermatocytes, they have recently 

 been extended to somatic cells. Here — doubtless in correlation with 

 the absence of a well-marked bouquet stage — the emission is diffuse, 



